With a view to improving the rolling resistance and to reducing fuel consumption, modern-day tires comprise rubber compounds which, by way of predominant filler content, contain electrically non-conducting fillers such as silica, which are used for example to make the tread strips. A tire of this type is described, by way of illustration, in publication EP 0 501 227.
Because of the very high resistivity of these compounds, their use has been accompanied by the development of numerous technical solutions for avoiding the build-up of static electricity, and for allowing charge to flow to the ground as the vehicle drives along. The disadvantages associated with the build-up of electrical charge are well known to equipment manufacturers, and have effects as diverse as disturbing the operation of the radio fitted to the vehicle, giving the occupant an electric shock as he/she gets out of the vehicle, or accelerated ageing of the tire as a result of the formation of ozone.
Hence, tire manufacturers have set themselves the task of bringing to market tires which do not have these disadvantages. Publication U.S. Pat. No. 5,518,055 describes a tire of which the tread strip, made of a non-conducting compound, is coated with a thin layer of conducting compound. This layer is in contact with the sidewall compounds, which are themselves also conductors of electricity, to allow electric charge to flow.
Another solution, set out in publication EP 0 658 452, involves placing an insert in the tread strip. This insert, which runs radially, preferably over the entire circumference of the tire, is made of an electrically conducting rubber compound and connects the external surface of the tread strip either to one of the crown reinforcing plies or to any other part of the tire that is contiguous with the tread strip and sufficiently electrically conducting, each of these plies being electrically conducting. Numerous improvements have been made to this principle, according to whether the tread strip comprises one or more layers of conducting or non-conducting materials, and these are set out by way of example in publications EP 0 925 903 or EP 0 963 302.
The object of all these methods is to connect the external surface of the tread strip with a part of the internal zone of the crown of the tire which is just adjacent to it, such as the sidewall, a crown reinforcing ply or carcass reinforcing ply, and which has properties of conducting electricity.
However, recent tire developments, again aimed at improving the rolling resistance, have led to a fairly widespread use of weakly electrically conducting compounds, based on silica, in most of the component parts of the tire liable to perform mechanical work during the running phase of the said tire. Thus, such compounds are used to produce the sidewalls, the carcass reinforcing plies, the crown reinforcing plies, or even the crown reinforcing profiled elements, in addition to their use in the tread strips.
Only those elements of the tire such as the bead reinforcing ring or the chafers, the function of which is to provide contact between the rim and the low region of the casing, are made using compounds containing a carbon-based filler, and still have the property of conducting electricity.